1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus for forming a toner image on a recording sheet by an electrophotographic process, such as an electrophotographic copying machine, printer, facsimile, etc.
2. Description of Related Art
Image forming apparatuses for forming toner images on recording sheets by an electrophotographic process have been developed into various types. FIG. 4 shows a model of such an image forming apparatus that the present inventors considered as a premise of the present invention. In the model shown by FIG. 4, a developing roller 42 for supporting toner in a layer, a first guide 25a, a second guide 25b and a transfer roller 34 are disposed around the circumference of a photosensitive drum 26 in this order in a direction shown by arrow “a”. A recording sheet is fed from a lower part in FIG. 4 and fed by a pair of timing rollers 24 through between the guides 25a and 25b, and a toner image is transferred onto the sheet while the sheet is being fed nipped between the photosensitive drum 26 and the transfer roller 34. Further, a fan (not shown) is disposed above the transfer roller 34 so as to suck up air around the photosensitive drum 26 and to exhaust the air to the outside.
The toner supported on the developing roller 42, which is shaped into a magnetic brush, flies and floats in the air by jumping-up and galling-over of the magnetic brush or by contact with the photosensitive drum 26 at a developing area D. The toner floating in the air is carried by airflows formed by rotations of the photosensitive drum 26, the developing roller 42 and the fan, and the toner flows along the circumference of the photosensitive drum 26, between the guides 25a and 25b and along the circumference of the transfer roller 34 as shown by arrows in FIG. 4. In this moment, the toner sticks to the guides 25a and 25b, which causes a problem that the recording sheet is begrimed with the toner stuck on the mutually opposite surfaces of the guides 25a and 25b. 
On the mutually opposite surfaces of the guides 25a and 25b, in an area where recording sheets of an often used size pass, toner is not deposited because toner is taken away from the area little by little every time a recording sheet of the often used size passes between the guides 25a and 25b. Outside the area, however, toner stuck on the mutually opposite surfaces of the guides 25a and 25b is not taken away unless a recording sheet of a large size passes between the guides 25a and 25b, and a large amount of toner is deposited there. Then, when a recording sheet of a large size passes between the guide 25a and 25b, the recording sheet is begrimed so heavily as can be seen. Additionally, the deposited toner may fall in lumps by vibration and may begrime the inside of the apparatus.
As a measure to prevent the inside of the image forming apparatus and recording sheets from being begrimed with toner flying from the developing device, Japanese Patent Laid-Open Publication No. 2002-278387 suggests that outside air be caused to flow in an airflow path between a developing device and a transfer guide by a fan to prevent the flying toner from depositing on the transfer guide. In order to implement this measure, however, components for forming the airflow path are necessary, thereby increasing the number of necessary components and increasing the cost. Also, as a measure to prevent recording sheets from being begrimed with toner and dust deposited on a protection lens of an exposure device, Japanese Patent Laid-Open Publication No. 5-45945 and Japanese Patent Laid-Open Publication No. 6-317951 suggest that an airflow be caused around the protection lens and that an air curtain be formed between the protection lens and a photosensitive drum. In order to implement this measure, however, a fan and a duct are necessary, thereby, increasing the number of components and increasing the cost.